Use part of an image sequence as specular map
Within Blender you can create a mask in the Image Editor, and then go into the compositor To Get This less saturated and rendered out with the Mask applied, then saved out as a Specular Map for this reason. I will play around with it, and append this answer with some screenshots/blend files/step-by-step instructions/etc...

Before I go much further, is this what you are asking for?: